1. Field of the Invention
This invention relates to an electronic annunciator system which is used to monitor a conductive loop which has a number of switch elements and a DC power source in series. More particularly, the invention relates to an encoder module which may be placed in parallel with each switch element to permit a remote identification of which switch elements have been activated in the monitored loop.
2. Description of the Prior Art
Conventional prior art security systems comprise a conductive loop which has a plurality of switch elements and a DC power source in series in the loop. The switch elements may be formed as frangible metal strips or other switching devices which are activated by a selected event. The switch elements may be activated by an intruder who is attempting to enter the protected premises or the switch elements may be temperature-activated to protect a premises from fire. In conventional systems, the interruption or opening of a switch element reduces the current flowing in the loop to zero which actuates a relay which sends an alarm signal over conventional telephone lines to a remote central office receiving station. These prior art systems may also actuate an alarm system on the premises protected by the security system. The principal defect of such alarm systems is that they do not permit a remote or on-site identification of which of the several possible switch elements has been opened.
An advance over this prior art is disclosed in U.S. Pat. No. 3,760,359 to McQuown. In the McQuown system, an impedance element such as a resistor is located in series with each switch device. Electronic circuitry is provided which measures the current flow through the circuit. Any attempt by an intruder to bypass this impedance element will result in either an increase or decrease in the supervisory current. Electronic circuitry which detects this change in current in the loop is used to initiate the operation of an oscillator which, when energized, places a signal on the alarm loop. The principal disadvantage of the McQuown invention is that only one oscillator at a time may be activated. Consequently, this prior art system cannot be used to indicate multiple intrusions or multiple switch openings.